Is there a difference between "pokoj" and "místnost" or are they interchangeable?
See https://forum.duolingo.com/comment/26071573/He-went-to-his-sister-s-room and https://forum.duolingo.com/comment/26035498
Why is "místnosti" is translated as "room", singular, in the mouse-over?
It is rooms now.
"I have three more rooms"?
Is this a question if this also works or are you asking something else?
No, it does not work:
I have = (já) mám
We have = (my) máme
Oh yeah I messed up with "I" and "We". Is "We have three more rooms" an accepted answer?
"We have three more rooms" is accepted, although I'd personally rather say "Máme další tři místnosti" for that.
Your explanation gives learners a glimpse of subtle shifts in meaning that happen when word order in a sentence is different. This is quite valuable. Thanks.